Fig. 4: Structural basis for efficient energy conversion in cyt bcc-aa₃ supercomplex.

Cofactors and substrate molecules of the supercomplex focusing on one protomer are shown with individual coloured icons. The grey line depicts the contour of the supercomplex. a Rapid electron transfer within the supercomplex with rate-limiting step between FeS and haem c_k, which is maintained by the fixed conformation of the subunits. Edge-to-edge distances between cofactors and calculated electron transfer rate constants (see Methods) are shown. The menaquinol position in the Q_o site (MK_o) was derived from bound stigmatellin. Detailed distances between MK_c and FeS are shown in Fig. 2a. Haem a, a₃ and Cu_B are strongly coupled in oxidases¹⁰⁰, thus rates for these electron transfer steps were omitted. The distance between the two haem b_L may facilitate intermonomer electron transfer as described for cyt bc₁ complex¹⁰⁸. b Q cycle of cyt bcc complex with fixed QcrA is enabled through two proton (H⁺) exit routes (Ex1, Ex2). QcrA is homologous to the mobile Rieske protein subunit of mitochondrial cyt bc₁ complex. En1 marks the proton uptake pathway to the Q_i site. The additional electron reservoir in Q_c site and lycopene (pictogram) can protect against the unproductive and deleterious radical formation. A static electron busbar provides a rapid electron transfer connection between the complexes through QcrA, QcrC and CtaC in fixed conformation (brown-lined box). Different conformational states of key protonable elements (highlighted in grey circles) provide a basis for rapid proton uptake through the D-channel, for proton loading into the exit route via conformational change of propionate δ of haem a₃ (PRD^a3) and for controlled proton release (Ex3) against proton motive force through coupled conformational states through a Cu_A ligand. Dioxygen and proton delivery to the binuclear centre might be coordinated through Glu267^CtaD conformational states. The surface of cyt bcc complex and aa₃ oxidase is shown as a grey line. The position of the membrane is indicated with dotted lines, with P and N denoting the periplasmic/electropositive and cytosolic/electronegative side, respectively.